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Nutrient loads from the land to the sea must be reduced to combat coastal eutrophication. It has been suggested that further mitigation efforts are needed in the brackish Baltic Sea to decrease nutrients, especially in eutrophic coastal areas. Mussel farming is a potential measure to remove nutrients directly from the sea.

Mussels consume phytoplankton containing nitrogen (N) and phosphorus (P); when the mussels are harvested these nutrients are removed from the aquatic system. However, sedimentation of organic material in faeces and pseudo-faeces below a mussel farm consumes oxygen and can lead to hypoxic or even anoxic sediments causing an increased sediment release of ammonium and phosphate. Moreover, N losses from denitrification can be reduced due to low oxygen and reduced numbers of bioturbating organisms. To reveal if mussel farming is a cost-effective mitigation measure in the Baltic Sea the potential for enhanced sediment nutrient release must be assessed.

Ocean Afforestation, more precisely Ocean Macroalgal Afforestation (OMA), has the potential to reduce atmospheric carbon dioxide concentrations through expanding natural populations of macroalgae, which absorb carbon dioxide, then are harvested to produce biomethane and biocarbon dioxide via anaerobic digestion. The plant nutrients remaining after digestion are recycled to expand the algal forest and increase fish populations.

A mass balance has been calculated from known data and applied to produce a life cycle assessment and economic analysis. This analysis shows the potential of Ocean Afforestation to produce 12 billion tons per year of biomethane while storing 19 billion tons of CO2 per year directly from biogas production, plus up to 34 billion tons per year from carbon capture of the biomethane combustion exhaust.These rates are based on macro-algae forests covering 9% of the world’s ocean surface, which could produce sufficient biomethane to replace all of today’s needs in fossil fuel energy, while removing 53 billion tons of CO2 per year from the atmosphere, restoring pre-industrial levels. This amount of biomass could also increase sustainable fish production to potentially provide 200 kg/yr/person for 10 billion people. Additional benefits are reduction in ocean acidification and increased ocean primary productivity and biodiversity.

Aspects of the nutrient-uptake physiology of Porphyra dioica (Brodie et Irvine) from Porto, Portugal were investigated under laboratory conditions. The capacity for uptake and accumulation of nitrogen (N) by P. dioica was determined for two different N sources, ammonium and nitrate (). The influence of the light–dark cycle and of the simultaneous presence of and , as well as the effects of phosphorus (P) enrichment, on the growth, nutrient uptake, and accumulation were also evaluated. Porphyra dioica was able to take up, accumulate, and grow equally well using both sources of nitrogen when presented separately. The photosynthetic pigment levels increased significantly with the increase of the availability of N, for both sources. The chlorophyll a content was higher in thalli that used as source of N, while this difference was not seen for phycobiliprotein content. When both N sources were available (NO3 : NH4 = 6 : 1), P. dioica preferentially removed , with a clear diurnal difference. During the light period, the algae removed 70% of the available, while only 35% was removed during the dark period. Phosphorus enrichment did not influence the growth rate or the amount of P removed from the medium, suggesting a limited capacity to store P. These results indicate that P. dioica is a good candidate for application in an integrated multi-trophic aquaculture (IMTA) system.

The nutritional values of protein concentrates (PCs) isolated from two subtropical red seaweeds, Hypnea charoides Lamouroux and H. japonica Tanaka, were evaluated in growing rats. The protein quality of the two seaweed PCs was determined by comparing the net protein ratio (NPR), true protein digestibility (TD), nitrogen balance (NB), biological value (BV), net protein utilization (NPU) and utilizable protein (UP) of the two seaweed PCs diet groups with those of the casein control group.

There were no significant differences of NPR and BV in all diet groups. Although the values of TD (ranged from 90.5 to 90.6%), NB (ranged from 108 to 113 mg rat−1 day−1), NPU (ranged from 80.1 to 81.3%) and UP (ranged from 80.1 to 81.3%) of these two PCs were significantly lower than those of the casein control, they were comparable to those of other common plant PCs. The growth performance of rats fed the two PCs diets was satisfactory and both PCs had no adverse effect on the weight of their major organs. Together with their good protein quality as mentioned above, the PCs from the two red seaweeds under study could be a potential alternative protein source for human nutrition.